The unique properties of laser radiation, which are related to its monochromaticity, coherence and, in pulsed form, its extremely high intensity, have stimulated much work with this form of energy. Interest in biological effects stems from applications in medicine, dentistry, and biology, and from health hazards associated with laser radiation. This research is concerned with the understanding of these effects at the molecular level and the use of laser techniques to reveal the finer details of photobiological processes. Versatile apparatus for nanosecond laser flash excitation has been developed. In this project it is used in conjunction with conventional flash photolysis and spectroscopic techniques to study the following specific problems. (1) Photoionization induced by laser excitation; (2) photochemical and photophysical properties of visual pigments; (3) photochemistry of flavins; (4) bilirubin photochemistry; (5) mechanisms of photodynamic effects; (6) multiphoton excitation of biologically significant molecules.